1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing toroidal cores comprising a plurality of layers of, for example, magnet iron, which extend through the aperture of another ring-like structure, such as an electrical coil of a transformer.
2. Description of the Related Art
It is generally known that electromagnetic induction devices such as transformers having a toroidally wound core encircling one or more wire-wound coils have high efficiency because they require less exciting current to establish a given flux as compared to other cores. Generally, in such devices, the magnet iron ribbon, from which the core is made, is threaded through one or more bobbins about which coil wire is wound.
FIGS. 1A through 1C depict an embodiment of a known transformer assembly which may be manufactured by the previous method and apparatus described immediately below, as well as by the inventive method and apparatus described further below. Transformer 20 comprises toroidal core 22 of ferrous strip stock material which links a pair of bobbins 24 and 26 respectively having center spool portions 28, 30 respectively disposed between flanges 32, 34 and 36, 38. Bobbins 22 and 24 may be identical and made from injection molded plastic. Spool portions 28, 30 are respectively provided with through-holes or apertures 40, 42. About each spool portion 28, 30, between its respective flanges, is wound a coil of wire (not shown) which may be covered with tape (as shown) or another suitable material. Referring to FIGS. 1A and 1B, the outer surface of the tape covering the wire about the spool portions is identified with reference numeral 44. The wire coils about spool portions 28 and 30 may have a different number of turns and be connected in series by means of connecting wires (not shown). Leading to and from the series-connected wire coils are a pair of electrical terminals 46; one terminal is connected to the start of one wire coil, the other terminal is connected to the end of the other wire coil. The bobbins may be completely wound with wire, the terminals attached, and the wires interconnected and taped prior to core 22 being wound through their apertures 40, 42 as discussed below.
Generally toroidal-shaped core, which extends through bobbin apertures 40, 42, encircles a portion of bobbins 24, 26, and may be made of a plurality of superimposed layers of magnet iron ribbon 48. Ribbon 48 is of uniform thickness and width, and has parallel opposite side edges 50, 52. Ribbon 48 may be provided in bulk to the manufacturing operation on spool 54 (FIG. 3). A continuous piece of ribbon 48 having leading end 56 is repeatedly wound through bobbin apertures 40, 42 in a spiral or flat coil shape, with the ribbon curved along its length. As shown in FIGS. 2A and 2B, leading end 56 has a leading edge 60, 60' which may be straight or beveled. The leading edge may also be arcuate (not shown). Beveled and curved leading edge configurations serve to help thread leading end 56 through bobbin apertures 40, 42. Once core 22 has been formed with the desired number of turns or layers of ribbon (e.g., 26 layers of ribbon 48), trailing end 58 is formed in the ribbon which comprises core 22. Trailing end 58 of ribbon 48 of each core 22 has a trailing edge. The trailing edge of a first core 22 is formed during a shearing process which simultaneously forms leading edge 60 of the next-to-be-manufactured core 22. Thus, the edges of the leading and trailing ends 56, 58 of a core 22 have complementary shapes.
Referring to FIGS. 1A, 1B and 3, it can be seen that in core 22, opposite first and second sides 62, 64 of ribbon 48 lay adjacent one another. Referring now in particular to FIGS. 1A and 1C, trailing end 58 is attached to the body of core 22 by means of being welded at 66 to the adjacent layer of ribbon 48. Weld 66 should not extend through the adjacent layer underlying the trailing end, and so plasma welding is preferred. Alternatively, however, trailing end 58 may be attached to core 22 by other means, such as being taped or clamped thereto. It should be noted that leading end 56 may remain free and unattached. As will be discussed below, in accordance with the previous method and apparatus for manufacture of the core, leading end 56 is urged into the inward most point of the spiral shape formed by ribbon 48 due to a plastic deformation of the ribbon itself.
Referring now to FIG. 3, there is shown previous apparatus 68, until now probably the most effective means for automated manufacture of devices such as transformers 20 which comprise toroidally-wound cores. Apparatus 68 comprises fixture 70 having first and second parts 72, 74. Within first fixture part 72 is passage 76 through which ribbon 48 is forced by means of pinch roller set 78. Ribbon 48 is pushed into fixture 70, and core 22 is formed therein as described hereinbelow. Ribbon 48 may be lubricated before entering fixture 70, or even pinch roller set 78, to ease its movement through passage 76, which is provided with sharp bend 80 therein. Lubrication of ribbon 48 may be especially beneficial where leading edge 60 has a bur thereon. As shown, passage bend 80 is approximately 90.degree., although other bend angles may be used instead. The purpose of bend 80 in passage 76 is to plastically deform ribbon 48 such that it takes on a permanent set which tends to urge the ribbon into a spiral or flat coil shape, with leading end 56 curving or spiraling inward. The resultant shape of the ribbon is a spiral or flat coil wherein the ribbon lies between two parallel planes which are perpendicular to ribbon sides 62, 64, and along which ribbon side edges 50, 52 lie. In conjunction with the structure of fixture 70 as further described hereinbelow, the plastic deformation ribbon 48 undergoes as it passes through bend 80 allows leading end 56 to be more easily directed initially through bobbin apertures 40, 42, and ribbon 48 itself to be more readily wound therethrough. Apparatus 68 is also provided with means such as a shear (not shown) for providing trailing end 58 in ribbon 48; this means may be located in fixture part 72 such that it severs ribbon 48 within passage 76.
Referring to FIG. 3, first and second fixture parts 72, 74 move relative to one another, with first fixture part 72 fixed and second fixture part 74 allowed to move in the directions of arrow 82. Spring 84 urges fixture part 74 into a first position in which expandible cavity 86 defined by and between fixture parts 72, 74 is at a first, smallest size. Fixture parts 72, 74 are each provided with respective cavity-forming arcuate walls 88, 90 of constant and identical radius of curvature, e.g., 0.800 inch.
Cavity 86 expands from its first, smallest size to a second, larger size by virtue of the movement of second fixture part 74 against the force of spring 84 as ribbon 48 fills the cavity and pushes the fixture parts away from each other, thereby expanding the size of cavity 86. Under this previous method, as increased amounts of ribbon 48 are fed into cavity 86, second fixture part 74 is forced away from first cavity part 72 in an uncontrolled manner.
First cavity part 72 is provided with recesses 92, 94 which partially define spaces 96, 98 in cavity 86. Bobbins 24, 26, which may be already wound with wire and connected thereby, are respectively disposed in spaces 96, 98. Passageway 76 extends into space 98 of cavity 86, directed towards aperture 42 of bobbin 26 and, as ribbon 48 is fed into cavity 86, it is threaded through the aperture of bobbin 26 and slidingly contacts arcuate wall 90 of second fixture part 74. During initial formation of core 22, ribbon 48 slidingly contacts arcuate wall 90 and, by means of its plastic deformation, which tends to spirally curl leading end 56 inward, and its contact with wall 90, leading end 56 is directed through aperture 40 of bobbin 24. The amount of plastic deformation of ribbon 48 induced by its being forced through corner 80 of passageway 76 may vary somewhat with strip stock variations and with the distance from the radial center of spool 54 at which the stock was stored on the spool.
Due to such variations in the amount of plastic deformation, and particularly as the number of turns or layers of ribbon 48 in core 22 increases, the plastic deformation of the ribbon may cause its leading edge 60 to come too close to the inward edge of the opening of bobbin aperture 40 or 42; consequently, leading end 56 may not always be fed through the bobbin aperture, instead sliding along flange 34 or 36 toward the center of cavity 86. When this occurs, the process is halted and the transformer being manufactured in fixture 70 is scrapped, compromising the consistency of product yield level from apparatus 68. A means of directing the ribbon through apertures 40, 42 of bobbins 24, 26 without plastic deformation thereof is therefore desirable, and would likely result in higher, and consistent, product yield levels.
Referring still to FIG. 3, after leading end 56 extends through bobbin 24, it comes into sliding contact with arcuate wall 88 of first fixture part 72. By means of the plastic deformation of ribbon 48 and its sliding contact with arcuate wall 88, leading end 56 is directed into aperture 42 of bobbin 26 again, wherein ribbon first and second ribbon sides 62, 64 interface and contact each other. Ribbon 48 is continually fed into fixture 70 and, as leading end 56 makes subsequent passes through apertures 40, 42 of bobbins 24 and 26, the number of ribbon layers in core 22 increases. As the number of ribbon layers in core 22 increases, second fixture part 74 is forced away from first fixture part 72, expanding cavity 86 against the force of spring 84. The expansion rate of cavity 86, although dependent on the amount of ribbon 48 in cavity 86, is uncontrolled. Consequently, the number of turns, or layers of ribbon 48, in a core 22 may undesirably vary. That is, although the amount of ribbon which has been fed into fixture 70 may be controlled, because the expansion of cavity 86 is dependent on how consistently the diametrical size of core 22 can be formed therein, the number of layers will vary: Cores having larger diameters will have fewer turns or layers as they are removed from the fixture, whereas cores having smaller diameters will have more turns. Notably, frictional resistance between adjacent ribbon sides 62, 64, or between ribbon side 64 and arcuate wall 88 of first cavity part 72, may affect the expansion rate of cavity 86. A means of better controlling the expansion of the chamber is desirable to produce cores of a consistent number of turns, thus improving the consistency of product quality.
After the desired amount of ribbon 48 has been fed into fixture 70, ribbon 48 is severed to provide trailing end 58 of the just-formed core 22 and leading end 56 of the next core 22 to be manufactured. The bobbin and core assembly is then moved to a subsequent welding or attaching station (not shown) where trailing end 58 may be then attached to the remainder of core 22 by, for example, providing weld 66 as described above. Further, a subsequent blocking station (not shown) may also be provided for then providing shoulders 100, 102, 104, 106 on core 22, as shown in FIG. 1A.
As indicated above, previous apparatus 68, although probably the most effective means known for automated manufacture of devices such as transformer 20 which comprise a toroidally-wound core, it is desirable to provide means for providing comparably higher and more consistent levels of product yield and quality.